One strand of the DNA is used as a template to create an mRNA copy. This process is called transcription. An mRNA copy needs to be made because DNA cannot leave the nucleus. Protein complexes separate the DNA helix to allow complementary mRNA nucleotides to bind to the DNA sequence. The pairing of nucleotides is very specific. Use the A, U, G and C buttons to place the correct mRNA nucleotide with each DNA nucleotide.